Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti3C2, the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti3AlC2 MAX. Since then, material scientists have either determined or predicted the stable phases of >200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electrochemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Applications of 2D MXenes in energy conversion and storage systems

2D transition metal carbides, carbonitrides, and nitrides, known as MXenes, are a rapidly growing family of 2D materials with close to 30 members experimentally synthesized, and dozens more studied theoretically. They exhibit outstanding electronic, optical, mechanical, and thermal properties with versatile transition metal and surface chemistries. They have shown promise in many applications, such as energy storage, electromagnetic interference shielding, transparent electrodes, sensors, catalysis, photothermal therapy, etc. The high electronic conductivity and wide range of optical absorption properties of MXenes are the key to their success in the aforementioned applications. However, relatively little is currently known about their fundamental electronic and optical properties, limiting their use to their full potential. Here, MXenes' electronic and optical properties from both theoretical and experimental perspectives, as well as applications related to those properties, are discussed, providing a guide for researchers who are exploring those properties of MXenes.

Electronic and Optical Properties of 2D Transition Metal Carbides and Nitrides (MXenes).

Heterocyclic pyrrole molecules are in situ aligned and polymerized in the -absence of an oxidant between layers of the 2D Ti3C2Tx (MXene), resulting in high volumetric and gravimetric capacitances with capacitance retention of 92% after 25,000 cycles at a 100 mV s(-1) scan rate.

Pseudocapacitive Electrodes Produced By Oxidant-Free Polymerization of Pyrrole Between the Layers of 2D Titanium Carbide (MXene)

Two-dimensional transition metal carbides and nitrides known as MXenes, with a general formula of Mn+1Xn (n = 1–3), integrate the advantages of metallic conductive transition metals with large groups of carbides, nitrides, or carbonitrides. They have led to a burgeoning research interest in biomedical applications due to their ultrathin structure and fascinating physiochemical (electronic, optical, magnetic, etc.) properties. In this review, we summarize recent advances in biomedical applications for MXenes. We first introduce the preparation methods and surface modifications with respect to MXenes. Their unique properties are then elaborated. Thirdly, we highlight their various biomedical applications, such as with biosensors, antibacterial materials, bioimaging probes, therapeutics, and theranostics. In the end, the current challenges and new opportunities for MXenes in regard to their biomedical applications are also discussed.

Two-dimensional transition metal carbides and nitrides (MXenes) for biomedical applications.

Owing to the exceptional properties of graphene, intensive studies have been carried out on novel two-dimensional (2D) materials. In the past several years, an elegant exfoliation approach has been used to successfully create a new family of 2D transition metal carbides, nitrides, and carbonitrides, termed MXene, from layered MAX phases. More recently, some unique properties of MXene have been discovered leading to proposals of potential applications. In this review, we summarize the latest progress in development of MXene from both a theoretical and experimental view, with emphasis on the possible applications.

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Recent advances in MXene: Preparation, properties, and applications

High voltage zinc varistors were synthesized by high-energy milling with Pr6O11 and Y2O3 doped ZnO-Bi2O3 system as raw materials. The effects of high-energy milling on the microstructures, electrical properties and densities of samples were investigated in detail. The results show that high-energy milling make the as-prepared varistors have refined and homogeneous microstructures, which result in improving their electrical properties. The optimal prepared samples were obtained by milling for 7.5 hours and sintering at 1100°C, which exhibited varistor voltage (V1mA ) of about 533 V/mm, non linear coefficient (α) reaching 36 and leakage current (IL ) of 2.96 μA.

Synthesis of Pr6O11 and Y2O3 Doped Zinc Varistors by High Energy Milling

The ternary layered machinable Ti3SiC2 ceramics were synthesized at 1300°C by high-energy milling and hot pressing sintering using Ti,Si and C powder as starting material. The reaction mechanism was examined by XRD, SEM and thermodynamic analysis. The results show that Ti-Si-C powders were transformed to Ti, Si and little TiSi, SiC,Ti5Si3 intermetallics by milled for 18 hours. In this process, the little Ti and C transformed to TiSi phase in early stage, and then the most of residual elements reacted to form SiC and Ti5Si3 intermetallics. Finally, Ti, Si, SiC, Ti5Si3 intermetallics and TiSi reacted to yield Ti3SiC2 by hot pressing sintering. And the sintering temperatures had a significant impact on the yielded content of Ti3SiC2 phase. Besides. The Ti3SiC2 would decompose if the temperature was high enough.

Synthesis of Ti3SiC2 Machinable Ceramics by High-Energy Milling and Hot-Pressing

The invention discloses an activated carbon loaded nanoscale N-doped cobalt phthalocyanine composite material and an in-situ solid-phase preparation method thereof and application of the activated carbon loaded nanoscale N-doped cobalt phthalocyanine composite material, and belongs to the technical field of preparation of Li/SOCl2 battery cathode catalytic materials. The activated carbon loaded nanoscale N-doped cobalt phthalocyanine composite material is synthesized in situ in a muffle furnace by using a solid-phase synthesis method. The method has the characteristics of the simple operation,the short period, the low energy consumption, the good repeatability, the high yield and the like. The activated carbon loaded nanoscale N-doped cobalt phthalocyanine composite material prepared by the method forms a composite nano material with a stable structure, a uniform size and good dispersity through Co-O coordination bonds and hydrogen bonds. When the material is used as a Li/SOCl2 battery cathode catalytic material, and under the condition that the discharge current density is 40mA/cm2, the discharge time is as long as 43 minutes and is 18 minutes longer than the discharge time of abattery without a catalyst, the voltage platform and the actual output specific energy are improved, and the catalyst can be used as a good cathode catalytic material of a Li/SOCl2 battery.

Activated carbon loaded nanoscale N-doped cobalt phthalocyanine composite material and in-situ solid-phase preparation method thereof and application of activated carbon loaded nanoscale N-doped cobalt phthalocyanine composite material

Sandstone monuments still standing after storms: Waterproofing and durability protection

Using Ti, Al and Cr2O3 as starting materials, Al2O3 particulate reinforced TiAl composites have been fabricated by in-situ reaction synthesis method. Effects of the Cr2O3 addition on the microstructures and mechanical properties of the TiAl/Al2O3 composites were investigated in detail. The results show that the composites have a matrix of TiAl, Ti3Al, and minor Cr containing phases, and a second reinforcement Al2O3. The addition of Cr2O3 effectively refined the structure of the matrix, and as a result, the mechanical properties of TiAl composites are improved. At Cr2O3 7.36 wt%, the flexural strength and fracture toughness reach the maximum values of 634.62 MPa and 9.79 MPa·m1/2, which are increased by 80% and 30%, respectively. The strengthening mechanism is also discussed.

Jianfeng Zhu

Papers

Synthesis of Pr6O11 and Y2O3 Doped Zinc Varistors by High Energy Milling

Synthesis of Ti3SiC2 Machinable Ceramics by High-Energy Milling and Hot-Pressing

Activated carbon loaded nanoscale N-doped cobalt phthalocyanine composite material and in-situ solid-phase preparation method thereof and application of activated carbon loaded nanoscale N-doped cobalt phthalocyanine composite material

Sandstone monuments still standing after storms: Waterproofing and durability protection

Influence of Cr2O3 Addition on the Properties of TiAl Composites